docs: def add_index_boost(self, index, boost):
          """Add a boost on an index.
  
          The Search object will be returned, so calls to this can be chained.
  
          """
          if boost is None:
              if index in self.index_boost:
                  del(self.index_boost[index])
          else:
              self.index_boost[index] = boost
          return self